Downgrade implicit casts to text to be assignment-only, except for the ones

from the other string-category types; this eliminates a lot of surprising
interpretations that the parser could formerly make when there was no directly
applicable operator.

Create a general mechanism that supports casts to and from the standard string
types (text,varchar,bpchar) for *every* datatype, by invoking the datatype's
I/O functions.  These new casts are assignment-only in the to-string direction,
explicit-only in the other, and therefore should create no surprising behavior.
Remove a bunch of thereby-obsoleted datatype-specific casting functions.

The "general mechanism" is a new expression node type CoerceViaIO that can
actually convert between *any* two datatypes if their external text
representations are compatible.  This is more general than needed for the
immediate feature, but might be useful in plpgsql or other places in future.

This commit does nothing about the issue that applying the concatenation
operator || to non-text types will now fail, often with strange error messages
due to misinterpreting the operator as array concatenation.  Since it often
(not always) worked before, we should either make it succeed or at least give
a more user-friendly error; but details are still under debate.

Peter Eisentraut and Tom Lane

3 files changed, 206 insertions, 106 deletions

diff --git a/src/backend/parser/parse_coerce.c b/src/backend/parser/parse_coerce.c
index c232dee4a58..98cc6691124 100644
--- a/src/backend/parser/parse_coerce.c
+++ b/src/backend/parser/parse_coerce.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/parse_coerce.c,v 2.153 2007/04/02 03:49:38 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/parse_coerce.c,v 2.154 2007/06/05 21:31:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -37,9 +37,10 @@ static Node *coerce_type_typmod(Node *node,
 				   bool hideInputCoercion);
 static void hide_coercion_node(Node *node);
 static Node *build_coercion_expression(Node *node,
-						  Oid funcId, bool arrayCoerce,
-						  Oid targetTypeId, int32 targetTypMod,
-						  CoercionForm cformat, bool isExplicit);
+									   CoercionPathType pathtype,
+									   Oid funcId,
+									   Oid targetTypeId, int32 targetTypMod,
+									   CoercionForm cformat, bool isExplicit);
 static Node *coerce_record_to_complex(ParseState *pstate, Node *node,
 						 Oid targetTypeId,
 						 CoercionContext ccontext,
@@ -121,8 +122,8 @@ coerce_type(ParseState *pstate, Node *node,
 			CoercionContext ccontext, CoercionForm cformat)
 {
 	Node	   *result;
+	CoercionPathType pathtype;
 	Oid			funcId;
-	bool		arrayCoerce;
 
 	if (targetTypeId == inputTypeId ||
 		node == NULL)
@@ -280,10 +281,11 @@ coerce_type(ParseState *pstate, Node *node,
 
 		return (Node *) param;
 	}
-	if (find_coercion_pathway(targetTypeId, inputTypeId, ccontext,
-							  &funcId, &arrayCoerce))
+	pathtype = find_coercion_pathway(targetTypeId, inputTypeId, ccontext,
+									 &funcId);
+	if (pathtype != COERCION_PATH_NONE)
 	{
-		if (OidIsValid(funcId) || arrayCoerce)
+		if (pathtype != COERCION_PATH_RELABELTYPE)
 		{
 			/*
 			 * Generate an expression tree representing run-time application
@@ -298,7 +300,7 @@ coerce_type(ParseState *pstate, Node *node,
 			baseTypeMod = targetTypeMod;
 			baseTypeId = getBaseTypeAndTypmod(targetTypeId, &baseTypeMod);
 
-			result = build_coercion_expression(node, funcId, arrayCoerce,
+			result = build_coercion_expression(node, pathtype, funcId,
 											   baseTypeId, baseTypeMod,
 											   cformat,
 										  (cformat != COERCE_IMPLICIT_CAST));
@@ -397,8 +399,8 @@ can_coerce_type(int nargs, Oid *input_typeids, Oid *target_typeids,
 	{
 		Oid			inputTypeId = input_typeids[i];
 		Oid			targetTypeId = target_typeids[i];
+		CoercionPathType pathtype;
 		Oid			funcId;
-		bool		arrayCoerce;
 
 		/* no problem if same type */
 		if (inputTypeId == targetTypeId)
@@ -426,8 +428,9 @@ can_coerce_type(int nargs, Oid *input_typeids, Oid *target_typeids,
 		 * If pg_cast shows that we can coerce, accept.  This test now covers
 		 * both binary-compatible and coercion-function cases.
 		 */
-		if (find_coercion_pathway(targetTypeId, inputTypeId, ccontext,
-								  &funcId, &arrayCoerce))
+		pathtype = find_coercion_pathway(targetTypeId, inputTypeId, ccontext,
+										 &funcId);
+		if (pathtype != COERCION_PATH_NONE)
 			continue;
 
 		/*
@@ -567,8 +570,8 @@ coerce_type_typmod(Node *node, Oid targetTypeId, int32 targetTypMod,
 				   CoercionForm cformat, bool isExplicit,
 				   bool hideInputCoercion)
 {
+	CoercionPathType pathtype;
 	Oid			funcId;
-	bool		arrayCoerce;
 
 	/*
 	 * A negative typmod is assumed to mean that no coercion is wanted. Also,
@@ -577,14 +580,15 @@ coerce_type_typmod(Node *node, Oid targetTypeId, int32 targetTypMod,
 	if (targetTypMod < 0 || targetTypMod == exprTypmod(node))
 		return node;
 
-	if (find_typmod_coercion_function(targetTypeId,
-									  &funcId, &arrayCoerce))
+	pathtype = find_typmod_coercion_function(targetTypeId, &funcId);
+
+	if (pathtype != COERCION_PATH_NONE)
 	{
 		/* Suppress display of nested coercion steps */
 		if (hideInputCoercion)
 			hide_coercion_node(node);
 
-		node = build_coercion_expression(node, funcId, arrayCoerce,
+		node = build_coercion_expression(node, pathtype, funcId,
 										 targetTypeId, targetTypMod,
 										 cformat, isExplicit);
 	}
@@ -609,6 +613,8 @@ hide_coercion_node(Node *node)
 		((FuncExpr *) node)->funcformat = COERCE_IMPLICIT_CAST;
 	else if (IsA(node, RelabelType))
 		((RelabelType *) node)->relabelformat = COERCE_IMPLICIT_CAST;
+	else if (IsA(node, CoerceViaIO))
+		((CoerceViaIO *) node)->coerceformat = COERCE_IMPLICIT_CAST;
 	else if (IsA(node, ArrayCoerceExpr))
 		((ArrayCoerceExpr *) node)->coerceformat = COERCE_IMPLICIT_CAST;
 	else if (IsA(node, ConvertRowtypeExpr))
@@ -630,7 +636,8 @@ hide_coercion_node(Node *node)
  */
 static Node *
 build_coercion_expression(Node *node,
-						  Oid funcId, bool arrayCoerce,
+						  CoercionPathType pathtype,
+						  Oid funcId,
 						  Oid targetTypeId, int32 targetTypMod,
 						  CoercionForm cformat, bool isExplicit)
 {
@@ -651,7 +658,7 @@ build_coercion_expression(Node *node,
 		/*
 		 * These Asserts essentially check that function is a legal coercion
 		 * function.  We can't make the seemingly obvious tests on prorettype
-		 * and proargtypes[0], even in the non-arrayCoerce case, because of
+		 * and proargtypes[0], even in the COERCION_PATH_FUNC case, because of
 		 * various binary-compatibility cases.
 		 */
 		/* Assert(targetTypeId == procstruct->prorettype); */
@@ -666,26 +673,7 @@ build_coercion_expression(Node *node,
 		ReleaseSysCache(tp);
 	}
 
-	if (arrayCoerce)
-	{
-		/* We need to build an ArrayCoerceExpr */
-		ArrayCoerceExpr *acoerce = makeNode(ArrayCoerceExpr);
-
-		acoerce->arg = (Expr *) node;
-		acoerce->elemfuncid = funcId;
-		acoerce->resulttype = targetTypeId;
-		/*
-		 * Label the output as having a particular typmod only if we are
-		 * really invoking a length-coercion function, ie one with more
-		 * than one argument.
-		 */
-		acoerce->resulttypmod = (nargs >= 2) ? targetTypMod : -1;
-		acoerce->isExplicit = isExplicit;
-		acoerce->coerceformat = cformat;
-
-		return (Node *) acoerce;
-	}
-	else
+	if (pathtype == COERCION_PATH_FUNC)
 	{
 		/* We build an ordinary FuncExpr with special arguments */
 		List	   *args;
@@ -723,6 +711,44 @@ build_coercion_expression(Node *node,
 
 		return (Node *) makeFuncExpr(funcId, targetTypeId, args, cformat);
 	}
+	else if (pathtype == COERCION_PATH_ARRAYCOERCE)
+	{
+		/* We need to build an ArrayCoerceExpr */
+		ArrayCoerceExpr *acoerce = makeNode(ArrayCoerceExpr);
+
+		acoerce->arg = (Expr *) node;
+		acoerce->elemfuncid = funcId;
+		acoerce->resulttype = targetTypeId;
+		/*
+		 * Label the output as having a particular typmod only if we are
+		 * really invoking a length-coercion function, ie one with more
+		 * than one argument.
+		 */
+		acoerce->resulttypmod = (nargs >= 2) ? targetTypMod : -1;
+		acoerce->isExplicit = isExplicit;
+		acoerce->coerceformat = cformat;
+
+		return (Node *) acoerce;
+	}
+	else if (pathtype == COERCION_PATH_COERCEVIAIO)
+	{
+		/* We need to build a CoerceViaIO node */
+		CoerceViaIO *iocoerce = makeNode(CoerceViaIO);
+
+		Assert(!OidIsValid(funcId));
+
+		iocoerce->arg = (Expr *) node;
+		iocoerce->resulttype = targetTypeId;
+		iocoerce->coerceformat = cformat;
+
+		return (Node *) iocoerce;
+	}
+	else
+	{
+		elog(ERROR, "unsupported pathtype %d in build_coercion_expression",
+			 (int) pathtype);
+		return NULL;			/* keep compiler quiet */
+	}
 }
 
 
@@ -1711,29 +1737,38 @@ IsBinaryCoercible(Oid srctype, Oid targettype)
  * find_coercion_pathway
  *		Look for a coercion pathway between two types.
  *
+ * Currently, this deals only with scalar-type cases; it does not consider
+ * polymorphic types nor casts between composite types.  (Perhaps fold
+ * those in someday?)
+ *
  * ccontext determines the set of available casts.
  *
- * If we find a suitable entry in pg_cast, return TRUE, and set *funcid
- * to the castfunc value, which may be InvalidOid for a binary-compatible
- * coercion.  Also, arrayCoerce is set to indicate whether this is a plain
- * or array coercion (if true, funcid actually shows how to coerce the
- * array elements).
+ * The possible result codes are:
+ *	COERCION_PATH_NONE: failed to find any coercion pathway
+ *				*funcid is set to InvalidOid
+ *	COERCION_PATH_FUNC: apply the coercion function returned in *funcid
+ *	COERCION_PATH_RELABELTYPE: binary-compatible cast, no function needed
+ *				*funcid is set to InvalidOid
+ *	COERCION_PATH_ARRAYCOERCE: need an ArrayCoerceExpr node
+ *				*funcid is set to the element cast function, or InvalidOid
+ *				if the array elements are binary-compatible
+ *	COERCION_PATH_COERCEVIAIO: need a CoerceViaIO node
+ *				*funcid is set to InvalidOid
  *
- * NOTE: *funcid == InvalidOid does not necessarily mean that no work is
+ * Note: COERCION_PATH_RELABELTYPE does not necessarily mean that no work is
  * needed to do the coercion; if the target is a domain then we may need to
  * apply domain constraint checking.  If you want to check for a zero-effort
  * conversion then use IsBinaryCoercible().
  */
-bool
+CoercionPathType
 find_coercion_pathway(Oid targetTypeId, Oid sourceTypeId,
 					  CoercionContext ccontext,
-					  Oid *funcid, bool *arrayCoerce)
+					  Oid *funcid)
 {
-	bool		result = false;
+	CoercionPathType result = COERCION_PATH_NONE;
 	HeapTuple	tuple;
 
 	*funcid = InvalidOid;
-	*arrayCoerce = false;
 
 	/* Perhaps the types are domains; if so, look at their base types */
 	if (OidIsValid(sourceTypeId))
@@ -1743,7 +1778,7 @@ find_coercion_pathway(Oid targetTypeId, Oid sourceTypeId,
 
 	/* Domains are always coercible to and from their base type */
 	if (sourceTypeId == targetTypeId)
-		return true;
+		return COERCION_PATH_RELABELTYPE;
 
 	/* Look in pg_cast */
 	tuple = SearchSysCache(CASTSOURCETARGET,
@@ -1779,7 +1814,10 @@ find_coercion_pathway(Oid targetTypeId, Oid sourceTypeId,
 		if (ccontext >= castcontext)
 		{
 			*funcid = castForm->castfunc;
-			result = true;
+			if (OidIsValid(*funcid))
+				result = COERCION_PATH_FUNC;
+			else
+				result = COERCION_PATH_RELABELTYPE;
 		}
 
 		ReleaseSysCache(tuple);
@@ -1789,7 +1827,7 @@ find_coercion_pathway(Oid targetTypeId, Oid sourceTypeId,
 		/*
 		 * If there's no pg_cast entry, perhaps we are dealing with a pair of
 		 * array types.  If so, and if the element types have a suitable cast,
-		 * report that with arrayCoerce = true.
+		 * report that we can coerce with an ArrayCoerceExpr.
 		 *
 		 * Hack: disallow coercions to oidvector and int2vector, which
 		 * otherwise tend to capture coercions that should go to "real" array
@@ -1798,25 +1836,30 @@ find_coercion_pathway(Oid targetTypeId, Oid sourceTypeId,
 		 * guaranteed to produce an output that meets the restrictions of
 		 * these datatypes, such as being 1-dimensional.)
 		 */
-		Oid			targetElemType;
-		Oid			sourceElemType;
-		Oid			elemfuncid;
-		bool		elemarraycoerce;
-
-		if (targetTypeId == OIDVECTOROID || targetTypeId == INT2VECTOROID)
-			return false;
-
-		if ((targetElemType = get_element_type(targetTypeId)) != InvalidOid &&
-			(sourceElemType = get_element_type(sourceTypeId)) != InvalidOid)
+		if (targetTypeId != OIDVECTOROID && targetTypeId != INT2VECTOROID)
 		{
-			if (find_coercion_pathway(targetElemType, sourceElemType,
-									  ccontext,
-									  &elemfuncid, &elemarraycoerce) &&
-				!elemarraycoerce)
+			Oid			targetElem;
+			Oid			sourceElem;
+
+			if ((targetElem = get_element_type(targetTypeId)) != InvalidOid &&
+				(sourceElem = get_element_type(sourceTypeId)) != InvalidOid)
 			{
-				*funcid = elemfuncid;
-				*arrayCoerce = true;
-				result = true;
+				CoercionPathType elempathtype;
+				Oid			elemfuncid;
+
+				elempathtype = find_coercion_pathway(targetElem,
+													 sourceElem,
+													 ccontext,
+													 &elemfuncid);
+				if (elempathtype != COERCION_PATH_NONE &&
+					elempathtype != COERCION_PATH_ARRAYCOERCE)
+				{
+					*funcid = elemfuncid;
+					if (elempathtype == COERCION_PATH_COERCEVIAIO)
+						result = COERCION_PATH_COERCEVIAIO;
+					else
+						result = COERCION_PATH_ARRAYCOERCE;
+				}
 			}
 		}
 
@@ -1826,14 +1869,39 @@ find_coercion_pathway(Oid targetTypeId, Oid sourceTypeId,
 		 * mistakenly conclude that ANYENUM-to-some-enum-type is a
 		 * trivial cast.
 		 */
-		if (!result)
+		if (result == COERCION_PATH_NONE)
 		{
 			if (type_is_enum(sourceTypeId))
 				result = find_coercion_pathway(targetTypeId, ANYENUMOID,
-											   ccontext, funcid, arrayCoerce);
+											   ccontext, funcid);
 			else if (sourceTypeId != ANYENUMOID && type_is_enum(targetTypeId))
 				result = find_coercion_pathway(ANYENUMOID, sourceTypeId,
-											   ccontext, funcid, arrayCoerce);
+											   ccontext, funcid);
+		}
+
+		/*
+		 * If we still haven't found a possibility, consider automatic casting
+		 * using I/O functions.  We allow assignment casts to textual types
+		 * and explicit casts from textual types to be handled this way.
+		 * (The CoerceViaIO mechanism is a lot more general than that, but
+		 * this is all we want to allow in the absence of a pg_cast entry.)
+		 * It would probably be better to insist on explicit casts in both
+		 * directions, but this is a compromise to preserve something of the
+		 * pre-8.3 behavior that many types had implicit (yipes!) casts to
+		 * text.
+		 */
+		if (result == COERCION_PATH_NONE)
+		{
+			if (ccontext >= COERCION_ASSIGNMENT &&
+				(targetTypeId == TEXTOID ||
+				 targetTypeId == VARCHAROID ||
+				 targetTypeId == BPCHAROID))
+				result = COERCION_PATH_COERCEVIAIO;
+			else if (ccontext >= COERCION_EXPLICIT &&
+					 (sourceTypeId == TEXTOID ||
+					  sourceTypeId == VARCHAROID ||
+					  sourceTypeId == BPCHAROID))
+				result = COERCION_PATH_COERCEVIAIO;
 		}
 	}
 
@@ -1851,19 +1919,26 @@ find_coercion_pathway(Oid targetTypeId, Oid sourceTypeId,
  *
  * If the given type is a varlena array type, we do not look for a coercion
  * function associated directly with the array type, but instead look for
- * one associated with the element type.  If one exists, we report it with
- * *arrayCoerce set to true.
+ * one associated with the element type.  An ArrayCoerceExpr node must be
+ * used to apply such a function.
+ *
+ * We use the same result enum as find_coercion_pathway, but the only possible
+ * result codes are:
+ *	COERCION_PATH_NONE: no length coercion needed
+ *	COERCION_PATH_FUNC: apply the function returned in *funcid
+ *	COERCION_PATH_ARRAYCOERCE: apply the function using ArrayCoerceExpr
  */
-bool
+CoercionPathType
 find_typmod_coercion_function(Oid typeId,
-							  Oid *funcid, bool *arrayCoerce)
+							  Oid *funcid)
 {
+	CoercionPathType result;
 	Type		targetType;
 	Form_pg_type typeForm;
 	HeapTuple	tuple;
 
 	*funcid = InvalidOid;
-	*arrayCoerce = false;
+	result = COERCION_PATH_FUNC;
 
 	targetType = typeidType(typeId);
 	typeForm = (Form_pg_type) GETSTRUCT(targetType);
@@ -1875,7 +1950,7 @@ find_typmod_coercion_function(Oid typeId,
 	{
 		/* Yes, switch our attention to the element type */
 		typeId = typeForm->typelem;
-		*arrayCoerce = true;
+		result = COERCION_PATH_ARRAYCOERCE;
 	}
 	ReleaseSysCache(targetType);
 
@@ -1893,5 +1968,8 @@ find_typmod_coercion_function(Oid typeId,
 		ReleaseSysCache(tuple);
 	}
 
-	return OidIsValid(*funcid);
+	if (!OidIsValid(*funcid))
+		result = COERCION_PATH_NONE;
+
+	return result;
 }
diff --git a/src/backend/parser/parse_expr.c b/src/backend/parser/parse_expr.c
index a0f92f66a64..45107e43ace 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/parse_expr.c,v 1.217 2007/04/27 22:05:48 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/parse_expr.c,v 1.218 2007/06/05 21:31:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -265,6 +265,7 @@ transformExpr(ParseState *pstate, Node *expr)
 		case T_FieldSelect:
 		case T_FieldStore:
 		case T_RelabelType:
+		case T_CoerceViaIO:
 		case T_ArrayCoerceExpr:
 		case T_ConvertRowtypeExpr:
 		case T_CaseTestExpr:
@@ -1806,6 +1807,9 @@ exprType(Node *expr)
 		case T_RelabelType:
 			type = ((RelabelType *) expr)->resulttype;
 			break;
+		case T_CoerceViaIO:
+			type = ((CoerceViaIO *) expr)->resulttype;
+			break;
 		case T_ArrayCoerceExpr:
 			type = ((ArrayCoerceExpr *) expr)->resulttype;
 			break;
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 01593317b1f..5e7a1cccffa 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/parse_func.c,v 1.195 2007/03/27 23:21:10 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/parse_func.c,v 1.196 2007/06/05 21:31:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -160,7 +160,7 @@ ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 	if (fdresult == FUNCDETAIL_COERCION)
 	{
 		/*
-		 * We can do it as a trivial coercion. coerce_type can handle these
+		 * We interpreted it as a type coercion. coerce_type can handle these
 		 * cases, so why duplicate code...
 		 */
 		return coerce_type(pstate, linitial(fargs),
@@ -669,7 +669,7 @@ func_select_candidate(int nargs,
  *	(exact match) is as quick as possible.
  *
  * If an exact match isn't found:
- *	1) check for possible interpretation as a trivial type coercion
+ *	1) check for possible interpretation as a type coercion request
  *	2) get a vector of all possible input arg type arrays constructed
  *	   from the superclasses of the original input arg types
  *	3) get a list of all possible argument type arrays to the function
@@ -720,29 +720,35 @@ func_get_detail(List *funcname,
 		 * If we didn't find an exact match, next consider the possibility
 		 * that this is really a type-coercion request: a single-argument
 		 * function call where the function name is a type name.  If so, and
-		 * if we can do the coercion trivially (no run-time function call
-		 * needed), then go ahead and treat the "function call" as a coercion.
+		 * if the coercion path is RELABELTYPE or COERCEVIAIO, then go ahead
+		 * and treat the "function call" as a coercion.
+		 *
 		 * This interpretation needs to be given higher priority than
 		 * interpretations involving a type coercion followed by a function
 		 * call, otherwise we can produce surprising results. For example, we
-		 * want "text(varchar)" to be interpreted as a trivial coercion, not
+		 * want "text(varchar)" to be interpreted as a simple coercion, not
 		 * as "text(name(varchar))" which the code below this point is
 		 * entirely capable of selecting.
 		 *
-		 * "Trivial" coercions are ones that involve binary-compatible types
-		 * and ones that are coercing a previously-unknown-type literal
-		 * constant to a specific type.
+		 * We also treat a coercion of a previously-unknown-type literal
+		 * constant to a specific type this way.
+		 *
+		 * The reason we reject COERCION_PATH_FUNC here is that we expect the
+		 * cast implementation function to be named after the target type.
+		 * Thus the function will be found by normal lookup if appropriate.
 		 *
-		 * The reason we can restrict our check to binary-compatible coercions
-		 * here is that we expect non-binary-compatible coercions to have an
-		 * implementation function named after the target type. That function
-		 * will be found by normal lookup if appropriate.
+		 * The reason we reject COERCION_PATH_ARRAYCOERCE is mainly that
+		 * you can't write "foo[] (something)" as a function call.  In theory
+		 * someone might want to invoke it as "_foo (something)" but we have
+		 * never supported that historically, so we can insist that people
+		 * write it as a normal cast instead.  Lack of historical support is
+		 * also the reason for not considering composite-type casts here.
 		 *
-		 * NB: it's important that this code stays in sync with what
-		 * coerce_type can do, because the caller will try to apply
-		 * coerce_type if we return FUNCDETAIL_COERCION.  If we return that
-		 * result for something coerce_type can't handle, we'll cause infinite
-		 * recursion between this module and coerce_type!
+		 * NB: it's important that this code does not exceed what coerce_type
+		 * can do, because the caller will try to apply coerce_type if we
+		 * return FUNCDETAIL_COERCION.  If we return that result for something
+		 * coerce_type can't handle, we'll cause infinite recursion between
+		 * this module and coerce_type!
 		 */
 		if (nargs == 1 && fargs != NIL)
 		{
@@ -755,16 +761,28 @@ func_get_detail(List *funcname,
 			{
 				Oid			sourceType = argtypes[0];
 				Node	   *arg1 = linitial(fargs);
-				Oid			cfuncid;
-				bool		arrayCoerce;
-
-				if ((sourceType == UNKNOWNOID && IsA(arg1, Const)) ||
-					(find_coercion_pathway(targetType, sourceType,
-										   COERCION_EXPLICIT,
-										   &cfuncid, &arrayCoerce) &&
-					 cfuncid == InvalidOid && !arrayCoerce))
+				bool		iscoercion;
+
+				if (sourceType == UNKNOWNOID && IsA(arg1, Const))
+				{
+					/* always treat typename('literal') as coercion */
+					iscoercion = true;
+				}
+				else
+				{
+					CoercionPathType cpathtype;
+					Oid			cfuncid;
+
+					cpathtype = find_coercion_pathway(targetType, sourceType,
+													  COERCION_EXPLICIT,
+													  &cfuncid);
+					iscoercion = (cpathtype == COERCION_PATH_RELABELTYPE ||
+								  cpathtype == COERCION_PATH_COERCEVIAIO);
+				}
+
+				if (iscoercion)
 				{
-					/* Yup, it's a trivial type coercion */
+					/* Treat it as a type coercion */
 					*funcid = InvalidOid;
 					*rettype = targetType;
 					*retset = false;