Fix #20502 - importc: macro conflicts with struct of same name (#21234)

Fixes https://github.com/dlang/dmd/issues/20502

Similar to VarDeclarations, don't let TemplateDeclarations from
C defines shadow a real symbol.

compiler/src/dmd/cparse.d

@@ -6057,6 +6057,8 @@ final class CParser(AST) : Parser!AST
             //printf("addSym() %s\n", s.toChars());
             if (auto v = s.isVarDeclaration())
                 v.isCmacro(true);       // mark it as coming from a C #define
+            if (auto td = s.isTemplateDeclaration())
+                td.isCmacro = true; // mark as coming from a C #define
             /* If it's already defined, replace the earlier
              * definition
              */

compiler/src/dmd/dtemplate.d

@@ -593,6 +593,7 @@ extern (C++) final class TemplateDeclaration : ScopeDsymbol
     bool isTrivialAliasSeq; /// matches pattern `template AliasSeq(T...) { alias AliasSeq = T; }`
     bool isTrivialAlias;    /// matches pattern `template Alias(T) { alias Alias = qualifiers(T); }`
     bool deprecated_;       /// this template declaration is deprecated
+    bool isCmacro;          /// Whether this template is a translation of a C macro
     Visibility visibility;
 
     // threaded list of previous instantiation attempts on stack

compiler/src/dmd/frontend.h

@@ -1749,6 +1749,7 @@ public:
     bool isTrivialAliasSeq;
     bool isTrivialAlias;
     bool deprecated_;
+    bool isCmacro;
     Visibility visibility;
     TemplatePrevious* previous;
     Expression* lastConstraint;

compiler/src/dmd/importc.d

@@ -544,6 +544,12 @@ Dsymbol handleSymbolRedeclarations(ref Scope sc, Dsymbol s, Dsymbol s2, ScopeDsy
         }
     }
 
+    // Don't let macros shadow real symbols
+    if (auto td = s.isTemplateDeclaration())
+    {
+        if (td.isCmacro) return s2;
+    }
+
     auto vd = s.isVarDeclaration(); // new declaration
     auto vd2 = s2.isVarDeclaration(); // existing declaration
 

compiler/src/dmd/template.h

@@ -71,6 +71,7 @@ public:
     d_bool isTrivialAliasSeq;     // matches `template AliasSeq(T...) { alias AliasSeq = T; }
     d_bool isTrivialAlias;        // matches pattern `template Alias(T) { alias Alias = qualifiers(T); }`
     d_bool deprecated_;           // this template declaration is deprecated
+    d_bool isCmacro;              // Whether this template is a translation of a C macro
     Visibility visibility;
 
     TemplatePrevious *previous;         // threaded list of previous instantiation attempts on stack

compiler/test/compilable/test20502.c

@@ -0,0 +1,10 @@
+// https://github.com/dlang/dmd/issues/20502
+struct mg_str {
+
+};
+
+void mg_str_s() {
+
+}
+
+#define mg_str(s) mg_str_s(s)