Fix Issue 4946 - Not good error message with wrongly positioned 'const'

src/ddmd/parse.d

@@ -4766,7 +4766,14 @@ final class Parser(AST) : Lexer
             }
             else if (!f.frequire && !f.fensure) // allow these even with no body
             {
-                error("semicolon expected following function declaration");
+                TOK t = token.value;
+                if (t == TOKconst || t == TOKimmutable || t == TOKinout || t == TOKreturn ||
+                    t == TOKshared || t == TOKnothrow || t == TOKpure)
+                    error("'%s' cannot be placed after a template constraint", token.toChars);
+                else if (t == TOKat)
+                    error("attributes cannot be placed after a template constraint");
+                else
+                    error("semicolon expected following function declaration");
             }
             break;
         }

test/fail_compilation/test4946.d

@@ -0,0 +1,19 @@
+/* TEST_OUTPUT:
+---
+fail_compilation/test4946.d(13): Error: 'pure' cannot be placed after a template constraint
+fail_compilation/test4946.d(14): Error: 'const' cannot be placed after a template constraint
+fail_compilation/test4946.d(15): Error: 'immutable' cannot be placed after a template constraint
+fail_compilation/test4946.d(16): Error: 'inout' cannot be placed after a template constraint
+fail_compilation/test4946.d(17): Error: 'shared' cannot be placed after a template constraint
+fail_compilation/test4946.d(18): Error: 'nothrow' cannot be placed after a template constraint
+fail_compilation/test4946.d(19): Error: attributes cannot be placed after a template constraint
+---
+*/
+
+void bar1(int x)() if (x > 0) pure { int a;}
+void bar2(int x)() if (x > 0) const { int a;}
+void bar3(int x)() if (x > 0) immutable { int a;}
+void bar4(int x)() if (x > 0) inout { int a;}
+void bar5(int x)() if (x > 0) shared { int a;}
+void bar6(int x)() if (x > 0) nothrow { int a;}
+void bar7(int x)() if (x > 0) @safe { int a;}